Self-assembly is an attractive approach for the integration of heterogeneous Microsystems. Various microcomponents can be independently batch microfabricated and then integrated onto a template via self-assembly using a variety of mechanisms including fluid flow, gravity, and electromagnetic forces. Recent advances in microfabrication and fluidic self-assembly are disclosed in U.S. patent application Ser. No. 12/305,365 which is hereby incorporated by reference in its entirety.
Fluidic self-assembly is a promising method to construct distributed active systems that cover large areas, for example. These systems may be assembled on flexible substrates with a variety of heterogeneous components. Applications for macroelectronics include smart artificial skins, large area phased-array radars, solar sails, flexible displays, electronic paper and distributed x-ray imagers. A candidate macrofabrication technology must be able to integrate a large number of various functional components over areas exceeding the size of a typical semiconductor wafer, in a cost-effective and time-efficient fashion.
Flexible plastic substrates are an attractive substrate for macroelectronic systems, but such plastics are typically thermally and chemically incompatible with conventional semiconductor fabrication processes. A number of approaches have been explored for low-temperature integration of a large number of semiconductor components onto a plastic substrate. The integration of the semiconductors is typically followed by a number of additional steps to build and interconnect functional devices. These material integration methods have demonstrated functional devices on plastic built from amorphous silicon, low temperature polysilicon, and a number of organic semiconductors.
An alternative approach for construction of macroelectronic systems is to perform the integration at the device level, instead of the material level. Significant infrastructure is available to cost-effectively fabricate high performance devices on single crystal semiconductor substrates. Even though recent advances in robotic assembly allow for positioning of up to 26,000 components per hour on plastic substrates, the relatively moderate speed, high cost, and limited positional accuracy of these systems make them unsuitable candidates for cost-effective mass production of macroelectronics.
A powerful technology that can meet all the criteria for an effective macrofabrication technology is self-assembly. In a device-level integration approach based on self-assembly, functional devices are batch microfabricated to yield a collection of freestanding components. These components are then manipulated such that at least some of the components self-assemble onto a template, for example onto a flexible plastic substrate, to yield a functional macroelectronic system. Self-assembly, utilized in the fashion outlined above, is an inherently parallel construction method that provides the potential for cost-effective and fast integration of a large number of functional components onto substrates, including unconventional substrates. For example, self-assembly may be suitable for the integration of microcomponents made by incompatible microfabrication processes (e.g., light emitting diodes made in compound semiconductor substrates versus silicon transistors) onto flexible substrates.
Self-assembly of micron-scale components and/or millimeter-scale components (“microcomponents”) have been studied previously both for two-dimensional and three-dimensional integration. In two-dimensional integration via self-assembly, a template with binding sites is prepared and a collection of parts is provided and manipulated to self-assemble onto the proper binding sites. The self-assembly procedure is typically performed in a liquid medium to allow for free motion of the microcomponents, and gravity and fluid dynamic forces are used to move the microcomponents and drive the system toward a minimum energy state.
A major drawback of prior art self-assembly methods has been the requirement for post-processing, for example the electrical interconnecting of the microcomponents after they have been self-assembled onto the template. Typically self-assembly of large numbers of components onto a substrate will produce a yield of less than 100% success, requiring additional post-processing to complete the assembly. In prior art methods, further processing of the substrate in a clean-room has been necessary to provide electrical connections and complete the assembly procedure. The need for extensive post-processing limits the applicability of prior art self-assembly methods when access to large areas and cost-effectiveness are determining factors. In order for the full potential of these techniques to be realized, batch microfabrication processes are needed to generate a large number of micron-scale functional components that can participate in self-assembly, and to increase the yield of the self-assembly process.